Oil fields typically have deposits of natural gas associated with them. In remote locations where transport of this gas may not be economically attractive, it has been common practice to burn it or re-inject it into the oil field. A relatively new alternative to burning and re-injection is gas conversion technology which offers the opportunity for chemically converting natural gas to higher molecular weight hydrocarbons. Current gas conversion technologies rely on the chemical conversion of natural gas to synthesis gas, a mixture of carbon monoxide and hydrogen, via partial oxidation or steam reforming. Synthesis gas is then reacted in a catalyzed hydrocarbon synthesis process commonly known as Fischer-Tropsch synthesis to form higher molecular weight hydrocarbons.
The preferred method of Fischer-Tropsch synthesis is carried out in a slurry bubble column reactor. This reactor is ideally suited for highly exothermic, three phase catalytic reactions and is described in U.S. Pat. No. 5,348,982. In such reactors, the solid phase catalyst, preferably cobalt, is dispersed or held in suspension in a liquid phase by a gas phase which continually bubbles through the liquid phase, thereby creating a three-phase mixture.
The main products from the Fischer-Tropsch process through the hydrogenation of carbon monoxide are Fischer-Tropsch waxes, which have many desirable properties. More specifically, they have very high purity being essentially free of any sulfur, nitrogen and aromatic species and have high normal paraffin content. Another desirable property of Fischer-Tropsch waxes is their opacity, i.e., their lack of translucent appearance. An additional very desirable property is that Fischer-Tropsch waxes are harder than conventional petroleum waxes. Fischer-Tropsch waxes produced through hydrogenation of carbon monoxide over cobalt catalysts yield harder Fischer-Tropsch waxes.
However, Fischer-Tropsch waxes produced at remote sites are difficult to transport to a location where they can be utilized or sold. Since these waxes are solids, they cannot be transported via conventional shipping or pipeline. Therefore, Fischer-Tropsch waxes are typically converted by reacting the Fischer-Tropsch wax with hydrogen over a hydroisomerization catalyst (U.S. Pat. No. 5,882,505) to form a liquid that can be pumped and hence, economically transported. This conversion represents a significant financial investment in gas conversion technologies. To reduce this investment, it is advantageous to combine raw Fischer-Tropsch wax product and crude oil to obtain a mixture that can be pumped and hence, economically transported from a remote site.
Wax and crude oil mixtures are typically produced by heating streams of wax and crude oil to a temperature above the melting point of the wax and subsequently mixing the two liquids together. However, during transport by pipeline such mixtures will cool resulting in the formation of crystallized networks of wax which will substantially increase the viscosity of the mixture. In addition, solid wax will form deposits along the walls of the pipeline in which crude oil is being transported, thereby decreasing the effectiveness of the pipeline and potentially causing damage to the pipeline structure. These consequences severely limit the amount of wax that can be blended into the crude oil. It is clearly desirable to increase the amount of wax that can be dispersed in crude oil since higher wax levels increase the overall volume of the blend that can be transported without significantly increasing the viscosity at lower temperatures. This objective is substantially achieved by the present invention.